In the field of mobile communication represented by cellular phone, it is becoming standard to handle a high-frequency signal of several-hundred MHz or above, and this inevitably requires a semiconductor device excellent in high-frequency characteristics. Semiconductor devices such as CMOS-IC or high-voltage-resistant IC, for example, use so-called SOI (Silicon on Insulator) wafer fabricated by forming a silicon oxide insulating layer on a silicon single crystal substrate (base wafer), and another silicon single crystal layer is stacked thereon as an SOI layer.
One known popular method of fabricating the SOI wafer is the bonding method. In the bonding method, a first silicon single crystal substrate as a base wafer, and a second silicon single crystal substrate (bond wafer) which configures an SOI layer in which devices will be formed, are bonded so as to oppose their mirror-polished surfaces while placing a silicon oxide film in between, and the bond wafer is then thinned to a predetermined thickness so as to make it into the SOI layer. Known methods for thinning the bond wafer include a method of grinding or polishing the back surface of the bond wafer after bonding, and a method of preliminarily implanting hydrogen ion or the like into the bond wafer and a portion thereof is delaminated after the bonding (ion implantation delamination method).
In the bonding step, the bond wafer and the base wafer in a stacked state are annealed so as to promote bonding. In this process, it is general that the outer circumferential portion of the wafer remains to some degree as an unbonded area due to polishing sag caused on the outer circumferential portion of the mirror-polished portion. The unbonded area is preferably as small as possible because it is a dead portion which cannot be used for producing final products. Several methods of reducing the unbonded area through improving accuracy in chamfering of the wafer are, however, still insufficient in the reducing effect, and further reduction even in a minimum degree is still a matter of large expectation.
This invention is conceived after considering the above-described demand, and a subject of which resides in reducing the unbonded area between the bond wafer and the base wafer in fabrication of a bonded wafer.